As inexpensive and small orientation sensors, like compasses, accelerometers, and gyroscopes, are widely available, they are being incorporated into many modern mobile devices, like cell phones, GPS receivers and others. The sensors are used to determine device orientation for portrait/landscape display switching, gesture recognition, map alignment in North direction, “point and click” object recognition, pedometry, navigation, etc.
While good orientation accuracy is important for most applications, achieving high accuracy with the low cost sensors is challenging. This is where sensor calibration often helps. The calibration process constitutes measuring and storing sensor parameters, like offset and scale factor. The calibrated parameters are then used in normal operation to accurately convert “raw” sensor readings into mobile device orientation.
Some mobile devices are being calibrated in factory in special fixtures, others by the user by placing the device in the specified orientations, while other devices undergo continuous or occasional automatic “on the fly” calibration during normal operation. Even if the device has been calibrated in factory, additional calibration, user or automatic, is useful and sometimes even unavoidable. For example, when a device featuring a magnetic compass is placed in a vehicle, magnetic fields induced by ferrous metal parts should be calibrated out to improve compass accuracy.